What is a thalweg?

The line of lowest elevation within a watercourse or valley is known as a thalweg or talweg (/ˈtɑːlvɛɡ/) in geography and fluvial geomorphology.

The word is of 19th-century German origin. It literally means "valley way."

In hydrology, the thalweg is a line drawn to join the lowest points along the entire length of a stream bed or valley in its downward slope, defining its deepest channel. The thalweg thus marks the natural direction (the profile) of a watercourse.

The thalweg principle is the legal principle that if the boundary between two political entities is a waterway, the boundary is the thalweg of that waterway. Notable examples include the Shatt al-Arab between Iraq and Iran, the Danube in central Europe, and the Kasikili/Sedudu Island dispute between Namibia and Botswana.